EL2157CS datasheet(14/19 Pages) INTERSIL

14 / 19 page

to the negative rail and split supplies are used, the output

range is extended to the negative rail.

Choice Of Feedback Resistor, RF

The feedback resistor forms a pole with the input

capacitance. As this pole becomes larger, phase margin is

reduced. This increases ringing in the time domain and

peaking in the frequency domain. Therefore, RF has some

maximum value which should not be exceeded for optimum

performance. If a large value of RF must be used, a small

capacitor in the few picofarad range in parallel with RF can

help to reduce this ringing and peaking at the expense of

reducing the bandwidth.

As far as the output stage of the amplifier is concerned, RF+

RG appear in parallel with RL for gains other than +1. As this

combination gets smaller, the bandwidth falls off.

Consequently, RF has a minimum value that should not be

exceeded for optimum performance.

For AV = +1, RF = 0Ω is optimum. For AV = -1 or +2 (noise

gain of 2), optimum response is obtained with RF between

500

Ω and 1kΩ. For AV = -4 or +5 (noise gain of 5), keep RF

between 2k

Ω and 10kΩ.

Video Performance

For good video performance, an amplifier is required to

maintain the same output impedance and the same

frequency response as DC levels are changed at the output.

This can be difficult when driving a standard video load of

150

Ω, because of the change in output current with DC

level. Differential Gain and Differential Phase for the EL2150

and EL2157 are specified with the black level of the output

video signal set to +1.2V. This allows ample room for the

sync pulse even in a gain of +2 configuration. This results in

dG and dP specifications of 0.05% and 0.05° while driving

150

Ω at a gain of +2. Setting the black level to other values,

although acceptable, will compromise peak performance.

For example, looking at the single supply dG and dP curves

for RL=150Ω, if the output black level clamp is reduced from

1.2V to 0.6V dG/dP will increase from 0.05%/0.05° to

0.08%/0.25° Note that in a gain of +2 configuration, this is

the lowest black level allowed such that the sync tip doesn't

go below 0V.

If your application requires that the output goes to ground,

then the output stage of the EL2150 and EL2157, like all

other single supply op amps, requires an external pull down

resistor tied to ground. As mentioned above, the current

flowing through this resistor becomes the DC bias current for

the output stage NPN transistor. As this current approaches

zero, the NPN turns off, and dG and dP will increase. This

becomes more critical as the load resistor is increased in

value. While driving a light load, such as 1k

Ω, if the input

black level is kept above 1.25V, dG and dP are a

respectable 0.03% and 0.03°.

For other biasing conditions see the Differential Gain and

Differential Phase vs Input Voltage curves.

Output Drive Capability

In spite of their moderately low 5mA of supply current, the

EL2150 and EL2157 are capable of providing ±100mA of

output current into a 10

Ω load, or ±60mA into 50Ω. With this

large output current capability, a 50

Ω load can be driven to

±3V with VS = ±5V, making it an excellent choice for driving

isolation transformers in telecommunications applications.

Driving Cables and Capacitive Loads

When used as a cable driver, double termination is always

recommended for reflection-free performance. For those

applications, the back-termination series resistor will de-

couple the EL2150 and EL2157 from the cable and allow

extensive capacitive drive. However, other applications may

have high capacitive loads without a back-termination

resistor. In these applications, a small series resistor (usually

between 5

Ω and 50Ω) can be placed in series with the

output to eliminate most peaking. The gain resistor (RG) can

then be chosen to make up for any gain loss which may be

created by this additional resistor at the output.

Disable/Power-Down

The EL2157 amplifier can be disabled, placing its output in a

high-impedance state. The disable or enable action takes

only about 40ns. When disabled, the amplifier's supply

current is reduced to 0mA, thereby eliminating all power

consumption by the EL2157. The EL2157 amplifier's power

down can be controlled by standard CMOS signal levels at

the ENABLE pin. The applied CMOS signal is relative to the

GND pin. For example, if a single +5V supply is used, the

logic voltage levels will be +0.5V and +2.0V. If using dual

±5V supplies, the logic levels will be -4.5V and

-3.0V. Letting the ENABLE pin float will disable the EL2157.

If the power-down feature is not desired, connect the

ENABLE pin to the VS+ pin. The guaranteed logic levels of

+0.5V and +2.0V are not standard TTL levels of +0.8V and

+2.0V, so care must be taken if standard TTL will be used to

drive the ENABLE pin.

Output Voltage Clamp

The EL2157 amplifier has an output voltage clamp. This

clamping action is fast, being activated almost

instantaneously, and being deactivated in <7ns, and

prevents the output voltage from going above the preset

clamp voltage. This can be very helpful when the EL2157 is

used to drive an A/D converter, as some converters can

require long times to recover if overdriven. The output

voltage remains at the clamp voltage level as long as the

product of the input voltage and the gain setting exceeds the

clamp voltage. If the EL2157 is connected in a gain of 2, for

example, and +3V DC is applied to the CLAMP pin, any

voltage higher than +1.5V at the inputs will be clamped and

+3V will be seen at the output.

EL2150, EL2157

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